1. Field of the Invention
The present invention relates to a high strength cold rolled steel sheet which has a high tensile strength of 40 Kgf/mm.sup.2 or higher and excellent non-aging property at room temperature and which is suitable for uses where specifically high press formability is required, e.g., automotive panels and the like, as well as in the production of hot-dip galvannealed steel sheet which is now facing an increasing demand, and also to a method for producing such a steel sheet.
The present invention also is concerned with a high strength cold rolled steel sheet which has a high tensile strength of 45 Kgf/mm.sup.2 or higher and excellent non-aging property at room temperature, as well as high bake hardenability (BH property) and which can suitably be used in the fields mentioned above, and also to a method of producing such a steel sheet.
In recent years, cold rolled steel sheets for drawing are required to meet the following requirements:
(1) greater strength to reduce both weight and cost while improving safety
(2) improved applicability to production of hot-dip galvannealed steel sheet which is light-weight and superior in corrosion resistance.
Various methods have been conventionally used for strengthening cold rolled steel sheet for working, typical examples of which are solid solution strengthening by addition of P and Mn, strengthening by formation of dual-phase structure of martensite and so forth, and precipitation strengthening caused by precipitation of Cu or like elements.
Application of steel sheets strengthened by solid-solution strengthening to drawing, however, is practically limited because such strengthening method causes a deterioration in workability. Further, addition of P which is the most effective element for strengthening the steel with minimum deterioration of workability conspicuously impedes zinc plating characteristic.
The strengthening by formation of the conventionally known dual-phase structure essentially requires addition of a comparatively large quantity of C, e.g., 0.05 to 1.0 wt %, in order to enable appearance of martensite and bainite as the second phase. Consequently, the steel sheet having the conventionally known dual-phase structure is not suitable for drawing, because the Lankford value (the r-value) conspicuously drops. In addition, martensite and bainite are undesirably annealed during galvannealing, which not only results in reduction of strength but allows generation of stretcher strain during forming. For these reasons, the steel sheets strengthened by the conventionally known dual-phase structure is not suitable for hot-dip galvannealing.
Precipitation strengthening tends to restrict conditions of production of steel sheets due to necessity for optimization of precipitation processing. In particular, production efficiency is seriously impaired when a precipitation treatment is additionally employed in the production process.
It has also been known that steel sheets can be hardened by aging caused by-accumulation of solid-solution C to dislocation which occurs during baked-on-finish, i.e., hardened by bake hardenability of the steel. In a strict sense, bake-hardening is different from precipitation-strengthening. The bake-hardened steel sheets, however, are widely used because the bake-hardening can be effected without substantially burdening the production process. Bake-hardening, however, provides only a small increase in the tensile strength, e.g., 1 to 2 Kgf/mm.sup.2 or so, although it improves tensile rigidity due to increase of yield strength by 3 to 5 Kgf/mm.sup.2. In addition, means are necessary for preventing aging before working or during plating. Thus, bake-hardening method also has disadvantages.
Consequently, known strengthening methods for strengthening steel sheets having high drawability have practical limits and steel sheets strengthened by such methods are not suitable for use as the material of hot-dip galvannealed steel sheets.
Under these circumstances, one of the present inventors, together with four other inventors, has proposed, in Japanese Patent Laid-Open No. 60-174852, a new type of cold rolled steel sheet and a method of producing the same, more specifically, a cold rolled steel sheet possessing excellent deep drawability and having a dual-phase structure composed of a ferrite phase and a low temperature transformed ferrite phase produced by annealing of extremely low carbon steel sheet in the temperature region where .alpha. and .gamma. phases coexist, as well as a process for producing such cold rolled steel sheet.
In contrast to known dual-phase-structure steel sheet having martensite and bainite as the second phase, the steel sheet proposed in Japanese Patent Laid-Open No. 60-174852 has the second phase constituted by low-temperature transformed ferrite having a high dislocation density.
The form of the low-temperature transformed ferrite varies according to the steel composition. According to an optical microscopic observation, the low-temperature transformed ferrite has one or a combination of two or more of the following three forms:
(1 ) crystal-like form with irregularly keened grain boundaries
(2 ) crystal grain-like form existing along grain boundaries in the same manner as precipitates
(3 ) crystal grain-like state or a state of a group of crystal grains (many sub-grain boundaries are found in comparatively large second phase grain) having a scratch-like form.
The low-temperature transformed ferrite, therefore, can be clearly distinguished from ordinary ferrite. In addition, the low-temperature transformed ferrite also can be clearly distinguished from martensite and bainite because the corroded portion inside the grain exhibits a color tone which is similar to that of ordinary ferrite and which is different from those of martensite and bainite.
On the other hand, an electron-microscopic observation reveals that the low-temperature transformed ferrite has a very high dislocation density in grain boundaries and/or grains. In particular, the low-temperature transformed ferrite in the third form (3) mentioned above exhibits a laminated structure having portions of extremely high dislocation density and comparatively low dislocation density.
In the steel sheet having the dual-phase structure composed of ferrite phase and low-temperature transformed ferrite phase as the second phase, the second phase is not annealed even when the steel is subjected to a high temperature of 550.degree. C., unlike the known cold rolled steel sheets having a second phase constituted by martensite or bainite which are easily annealed. The steel having the above-mentioned dual-phase structure, therefore, is suitable for use as the material of hot-dip galvannealed steel sheets.
The steel sheet having the above-mentioned dual-phase structure also is superior in that the r-value is much higher than those of steel sheets having conventional dual-phase structure, due to the fact that the matrix phase is constituted by extremely-low carbon ferrite which has been recrystallized at ordinary high temperature. In addition, this steel sheet simultaneously exhibits both high bake hardenability and non-aging property at room temperature, because the dual-phase structure has internal local strain.
The strengthening effect produced by low-temperature transformed ferrite is not so remarkable as compared with the effect produced by martensite or bainite. In order to further strengthen the steel sheet, therefore, it is necessary to add strengthening elements such as Mn, Nb and B. Addition of such elements to the steel of the kind described, however, tends to deteriorate workability and extremely restricts the range of annealing temperature which would provide good workability, with the result that the production efficiency is lowered.